Zinc Phosphides as Outstanding Sodium-Ion Battery Anodes

To design a high-performance sodium-ion battery anode, binary zinc phosphides (ZnP₂ and Zn₃P₂) were synthesized by a facile solid-state heat treatment process, and their Na storage characteristics were evaluated. The Na reactivity of ZnP₂ was better than that of Zn₃P₂. Therefore, a C-modified ZnP₂-based composite (ZnP₂-C) was fabricated to achieve better electrochemical performance. To investigate the electrochemical reaction mechanism of ZnP₂-C during sodiation/desodiation, various ex situ analytical techniques were employed. During sodiation, ZnP₂ in the composite was transformed into NaZn₁₃ and Na₃P phases, exhibiting a one-step conversion reaction. Conversely, Zn and P in NaZn₁₃ and Na₃P, respectively, were fully recombined to the original ZnP₂ phase during desodiation. Owing to the one-step conversion/recombination of ZnP₂ in the composite during cycling, the ZnP₂-C showed high electrochemical performance with a highly reversible capacity of 883 mA h g^-1 after 130 cycles with no capacity deterioration and a fast C-rate capability of 500 mA h g^-1 at 1 C and 350 mA h g^-1 at 3 C.